Non-Ferroelectric High Dielectric and Preparation Method Thereof

ABSTRACT

Provided is a method for preparing a grain boundary insulation-type dielectric. The method includes the steps of obtaining a titanic acid compound and a ferroelectric having a value less than a melting point of the titanic acid compound; obtaining a mixture by adding the ferroelectric material to the titanic acid compound; and sintering the mixture at a temperature equal to or more than a melting point of the ferroelectric material.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Korean Patent Application No.10-2017-0058496 filed May 11, 2017, the disclosure of which is herebyincorporated in its entirety by reference.

TECHNICAL FIELD

The present invention relates to a non-ferroelectric high electric and apreparation method thereof, and more particularly, to a grain boundaryinsulation-type dielectric consisting of semiconducting titanic acidcompound particles and insulating grain boundaries, and a preparationmethod thereof.

BACKGROUND ART

A dielectric material is a material in which polarization is generatedwhen an electric field is applied thereto, and is used in electronicdevices because the dielectric material serves to stabilize power supplyby storing a predetermined amount of electricity when the dielectricmaterial is used as a capacitor and alleviate an influx of sparks into acircuit in an alternating current power supply.

Existing ceramic capacitors are classified into Class I and II. Class Iis for temperature compensation, has a very small capacitance changerate according to the temperature, and good high-frequencycharacteristics, and uses a paraelectric such as (Ca,Sr)(Ti,Zr)O₃. ClassII is for temperature compensation and has an aspect in which a changein dielectric constant according to the temperature is large and changewidths in dielectric constant and dielectric loss under alternating anddirect current voltages are large, but has a high dielectric constantvalue and uses a ferroelectric such as (Ba,Ca)(Ti,Zr)O₃.

In the early period of development of ceramic capacitors in order todevelop a high capacitance capacitor, studies using Pb(Ti, Zr)O₃ havinga relative dielectric constant of approximately 200,000 as a basematerial have been conducted, but due to problems in that lead ishazardous to the environment and human bodies, studies using(Ba,Ca)(Ti,Zr)O₃ which is Class II for a high dielectric constant, andthe like as a base material have been actively conducted.

Currently, in order to improve the relative dielectric constant andenhance temperature stability, studies in which various additives aremixed or a core-shell structure is formed or subjected to grain boundarysegregation during a heat treatment process after additives are mixed,or studies in which a heat treatment is performed by chemically coatingan initial powder, and then preparing a molded body have been mainlyconducted.

Recently, as electronic devices have been rapidly reduced in size due tothe development of technology, industrially used capacitors are greatlyrequired to have high capacitance and achieve reduction in size. Thatis, there is a need for developing a dielectric material which is usedin a stack-type ceramic capacitor due to the high relative dielectricconstant, or has small particles for reduction in size of a capacitor.

In a grain boundary insulation-type capacitor consisting ofsemiconducting particles and insulating grain boundaries, it is assumedthat capacitors at the grain boundary are connected in series. In thiscase, a capacitance, which is a physical quantity exhibiting an abilityof an object to accumulate electric charge, is a value obtained bydividing the number of capacitors connected in series in the capacitanceof a grain boundary. When the thickness of a sample is d_(c), the sizeof a particle is d_(b), the thickness of a grain boundary is d_(gb), therelative dielectric constant of the grain boundary is co, and thesurface area is A, the number of capacitors connected in series, n is

${{n = \frac{\text{?}}{d_{b}}},{\text{?}\text{indicates text missing or illegible when filed}}}\mspace{340mu}$

and

the capacitance of a grain bound, C_(gb) is

${Cgb} = {\epsilon_{0}\epsilon_{gb}{\frac{A}{d_{{ob}\;}}.}}$

Accordingly, the total capacitor of n capacitors of the capacitanceC_(gb), which are connected in series, is

$\begin{matrix}{C = \frac{C_{gb}}{n}} \\{{= {\frac{\epsilon_{0}\epsilon_{gb}A}{d_{gb}}\frac{d_{b}}{\text{?}}}},}\end{matrix}$ ?indicates text missing or illegible when filed

and

the apparent dielectric constant, ε_(app) is

$\epsilon_{app} = {\epsilon_{gb}{\frac{d_{b}}{d_{gb}}.}}$

In order for the capacitor to achieve reduction in size and have a highcapacitance, the particle size (d_(b)) needs to be small, but when theparticle size becomes small, there is a problem in that the apparentdielectric constant (ε_(app)) also becomes small.

SUMMARY OF THE INVENTION

The present invention has been made in an effort to solve theaforementioned problems, and an object thereof is to provide anon-ferroelectric high electric which has a high relative dielectricconstant even in small particle sizes, and has a narrow change width indielectric constant vs. the temperature, and a high relative dielectricconstant and a low dielectric loss even in a high frequency region, anda preparation method thereof.

An exemplary embodiment of the present invention provides a method forpreparing a grain boundary insulation-type dielectric, the methodincluding: obtaining a titanic acid compound and a ferroelectric havinga value less than a melting point of the titanic acid compound;obtaining a mixture by adding the ferroelectric material to the titanicacid compound; and sintering the mixture at a temperature equal to ormore than a melting point of the ferroelectric material.

The titanic acid compound is characterized by being SrTiO₃.

The titanic acid compound is characterized by being(Sr_(x)A_(y))Ti_(z)O₃. (Here, A is an element having a valence of 3 ormore, 0.95≤x≤0.99, 0.01≤y≤0.05, 1.00≤z≤1.01, and x+y=1.)

The titanic acid compound is characterized by being(Sr_(x)La_(y))Ti_(z)O₃. (Here, 0.95≤x≤0.99, 0.01≤y≤0.05, 1.00≤z≤1.01,and x+y=1.)

The sintering may include sintering under a reducing atmosphere and asubsequent heat treatment under an oxidizing atmosphere, and thesubsequent heat treatment under the oxidizing atmosphere ischaracterized by being performed under normal pressure.

The titanic acid compound is characterized by being may be BaTiO₃.

The titanic acid compound is characterized by being(Ba_(x)A_(y))Ti_(z)O₃. (Here, A is an element having a valence of 3 ormore, 0.95≤x≤0.99, 0.01≤y≤0.05, 1.00≤z≤1.01, and x+y=1.)

The titanic acid compound is characterized by being(Ba_(x)La_(y))Ti_(z)O₃. (Here, 0.95≤x≤0.99, 0.01≤y≤0.05, 1.00≤z≤1.01,and x+y=1.)

The method is characterized by further including adding tetraethylorthosilicate (TEOS) to the mixture of the titanic acid compound and theferroelectric.

The method is characterized by further including subjecting the mixtureto pre-heat treatment (prefiring) prior to the sintering.

The sintering is characterized by including sintering under a reducingatmosphere and a subsequent heat treatment under an oxidizingatmosphere, and the subsequent heat treatment under the oxidizingatmosphere is characterized by being performed under an N₂ atmosphere ornormal pressure.

An addition ratio of the ferroelectric is characterized by being 2 to 20mol % based on the titanic acid compound.

The ferroelectric material is characterized by being ABO₃, which has aperovskite structure. (Here, A is any one of K, Na and K_(0.5)Na_(0.5),and B is any one of Nb and Ta.)

A ferroelectric having an ABO₃ structure in (Sr_(x)La_(y))Ti_(z)O₃ whichis a strontium titanate compound is characterized by being distributedat a grain boundary of the strontium titanate compound. (Here,0.95≤x≤0.99, 0.01≤y≤0.05, 1.00≤z≤1.01, x+y=1, A is any one of K, Na andK_(0.5)Na_(0.5), and B is any one of Nb and Ta.)

The grain boundary insulation-type dielectric is characterized by havingan average particle size of 0.3 μm to 1 μm.

When the ferroelectric is K_(0.5)Na_(0.5)NbO₃, the grain boundaryinsulation-type dielectric is characterized by having a relativedielectric constant of 4,500 to 6,000 and a dielectric loss of 2 to 5%in a frequency region of 1 MHz or more.

When the ferroelectric is KNbO₃, the grain boundary insulation-typedielectric is characterized in that a change width in relativedielectric constant is maintained at 0 to 10 and a change width indielectric loss is maintained at 0 to 5%, regardless of the frequencyregion.

A ferroelectric having an ABO₃ structure in (Ba_(x)La_(y))Ti_(z)O₃ whichis a barium titanate compound is characterized by being distributed at agrain boundary of the barium titanate compound. (Here, 0.95≤x≤0.99,0.01≤y≤0.05, 1.00≤z≤1.01, x+y=1, A is any one of K, Na andK_(0.5)Na_(0.5), and B is any one of Nb and Ta.)

The grain boundary insulation-type dielectric is characterized by havingan average particle size of 0.2 μm to 1 μm.

When the ferroelectric is K_(0.5)Na_(0.5)NbO₃, the grain boundaryinsulation-type dielectric is characterized by having a relativedielectric constant of 1,400 to 3,200 and a dielectric loss of 10 to 20%in a frequency region of 1 MHz or more.

The grain boundary insulation-type dielectric is characterized in that achange width in relative dielectric constant is maintained at 0 to 20and a change width in dielectric loss is maintained at 0 to 2%,regardless of the frequency region.

A ratio of the ferroelectric in the grain boundary insulation-typedielectric is characterized by being 2 to 20 mol % based on the titanicacid compound.

The present invention is a grain boundary insulation-type dielectricconsisting of titanic acid compound-based particles having a size ofless than 1 μm and a ferroelectric grain boundary, and a dielectricbased on a strontium titanate compound exhibits a high relativedielectric constant and a low dielectric loss in spite of small particlesizes and has a small change width in the dielectric constant accordingto the change in temperature. Accordingly, the present invention issuitable for a high capacitance capacitor, and may reduce the size of anelectric part.

In the case of a dielectric based on a barium titanate compound, when asubsequent heat treatment is performed under N₂, the dielectric has ahigh relative dielectric constant of 1,400 or more and a dielectric lossof 20% or less in a high frequency region of 1 MHz. In contrast, when asubsequent heat treatment is performed under normal pressure (air), thedielectric has a constant relative dielectric constant and thedielectric loss is also maintained at a low level, almost regardless offrequency.

Since ferroelectric materials (K_(0.5)Na_(0.5)NbO₃, KNbO₃, and NaNbO₃)non-hazardous to the environment and human bodies are used, the presentinvention is more eco-friendly than existing dielectrics.

However, the effects which the non-ferroelectric high electric and thepreparation methods thereof according to exemplary embodiments of thepresent invention can achieve are not limited to those mentioned above,and the other effects not mentioned will be clearly understood by aperson with ordinary skill in the art to which the present inventionpertains from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings included as a part of the detailed descriptionto assist understanding of the present invention provide exemplaryembodiments of the present invention and explain the technical spirit ofthe present invention along with the detailed description.

FIG. 1 is a flow chart explaining a method for preparing a grainboundary insulation-type dielectric according to an exemplary embodimentof the present invention.

FIG. 2 is a schematic view illustrating a sintering process of the grainboundary insulation-type dielectric according to an exemplary embodimentof the present invention.

FIG. 3 is a scanning electron microscope image illustrating particlesizes of a (Sr_(0.95)La_(0.05))Ti_(1.01)O₃ compound according to anexemplary embodiment of the present invention and a raw material powder.

FIG. 4 is a flowchart illustrating a preparation method when a titanicacid compound is a barium titanate compound, in the grain boundaryinsulation-type dielectric according to an exemplary embodiment of thepresent invention.

FIG. 5 is a scanning electron microscope image illustrating the microstructures of the samples of (100-x)(Sr_(0.95)La_(0.05))Ti_(1.01)O₃-xKNNaccording to an exemplary embodiment of the present invention.

FIG. 6 is a scanning electron microscope image of(Ba_(0.95)La_(0.05))Ti_(1.01)O₃ powder prepared through a calcinationprocess according to an exemplary embodiment of the present invention.

FIG. 7 is a scanning electron microscope image illustrating changes inmicro structures according to the content of TEOS in90(Ba_(0.95)La_(0.05))Ti_(1.01)O₃-10KNN according to an exemplaryembodiment of the present invention.

FIG. 8A is a graph illustrating changes in relative dielectric constantvalues according to the frequency measured in(100-x)(Sr_(0.95)La_(0.05))Ti_(1.01)O₃-xKNN according to the presentinvention, and FIG. 8B is a graph illustrating changes in dielectricloss values.

FIG. 9A is a graph illustrating changes in relative dielectric constantvalues according to the frequency measured in(100-x)(Sr_(0.95)La_(0.05))Ti_(1.01)O₃-xKN according to the presentinvention, and FIG. 9B is a graph illustrating changes in dielectricloss values.

FIG. 10A is a graph illustrating changes in relative dielectric constantvalues according to the frequency measured in (100-x)BT-xKNN in which anoxidizing atmosphere of a subsequent heat treatment process consists ofan N₂ atmosphere in (100-x)BT-xKNN, and FIG. 10B is a graph illustratingchanges in dielectric loss values.

FIG. 11A is a graph illustrating changes in relative dielectric constantvalues according to the addition of TEOS in 90BT-10KNN in which anoxidizing atmosphere of a subsequent heat treatment process consists ofan N₂ oxidizing atmosphere in (100-x)BT-xKNN, and FIG. 11B is a graphillustrating changes in dielectric loss values.

FIG. 12A is a graph illustrating changes in relative dielectric constantvalues when (100-x)BT-xKNN is subjected to pre-heat treatment(prefiring) as a pre-step of sintering in 90BT-10KNN in which anoxidizing atmosphere of a subsequent heat treatment process consists ofan N₂ oxidizing atmosphere in (100-x)BT-xKNN, and FIG. 12B is a graphillustrating changes in dielectric loss values.

FIG. 13A is a graph illustrating changes in relative dielectric constantvalues of 90BT-10KNN according to the sintering temperature performedunder a reducing atmosphere when a subsequent heat treatment process isperformed under an N₂ oxidizing atmosphere in (100-x)BT-xKNN, and FIG.13B is a graph illustrating changes in dielectric loss values.

FIG. 14A is a graph illustrating changes in relative dielectric constantvalues according to the frequency measured in (100-x)BT-xKNN in which anoxidizing atmosphere of a subsequent heat treatment process consists ofa normal pressure (air) oxidizing atmosphere in (100-x)BT-xKNN, and FIG.14B is a graph illustrating changes in dielectric loss values.

FIG. 15A is a graph illustrating changes in relative dielectric constantvalues according to the addition of TEOS in 90BT-10KNN in which anoxidizing atmosphere of a subsequent heat treatment process consists ofa normal pressure (air) oxidizing atmosphere in (100-x)BT-xKNN, and FIG.15B is a graph illustrating changes in dielectric loss values.

FIG. 16A is a graph illustrating changes in relative dielectric constantvalues when (100-x)BT-xKNN is subjected to pre-heat treatment(prefiring) as a pre-step of sintering in 90BT-10KNN in which anoxidizing atmosphere of a subsequent heat treatment process consists ofa normal pressure (air) oxidizing atmosphere in (100-x)BT-xKNN, and FIG.16B is a graph illustrating changes in dielectric loss values.

FIG. 17A is a graph illustrating changes in relative dielectric constantvalues of 90BT-10KNN according to the sintering temperature under areducing atmosphere when a subsequent heat treatment process isperformed under normal pressure (air) oxidizing atmosphere in(100-x)BT-xKNN, and FIG. 17B is a graph illustrating changes indielectric loss values.

DETAILED DESCRIPTION

The terms or words used in the present specification and the claimsshould not be construed as being limited as typical or dictionarymeanings, and should be construed as meanings and concepts conforming tothe technical spirit of the present invention on the basis of theprinciple that an inventor can appropriately define concepts of theterms in order to describe his or her own invention in the best way.Accordingly, since the exemplary embodiments described in the presentspecification and the configurations illustrated in the drawings areonly the most preferred exemplary embodiment of the present inventionand do not represent all of the technical spirit of the presentinvention, it is to be understood that various equivalents and modifiedembodiments, which may replace these exemplary embodiments andconfigurations, are possible at the time of filing the presentapplication. Hereinafter, a non-ferroelectric high electric and apreparation method thereof according to an exemplary embodiment of thepresent invention will be described in detail with reference to theaccompanying drawings.

FIG. 1 is a flow chart explaining a method for preparing a grainboundary insulation-type dielectric according to an exemplary embodimentof the present invention, and FIG. 2 is a schematic view illustrating asintering process of the grain boundary insulation-type dielectricaccording to an exemplary embodiment of the present invention.

Referring to FIG. 1, a method for preparing a grain boundaryinsulation-type dielectric will be described. A method for preparing agrain boundary insulation-type dielectric according to the presentinvention is characterized by including: obtaining a titanic acidcompound and a ferroelectric having a value less than a melting point ofthe titanic acid compound (S100); obtaining a mixture by adding theferroelectric material to the titanic acid compound (S110); andsintering the mixture at a temperature equal to or more than a meltingpoint of the ferroelectric material (S120).

Step S100 will be described. The titanic acid compound (powder) isprepared by wet-milling, drying, grinding, and sieving a raw materialpowder, and calcining the resulting product. A ferroelectric materialhas a perovskite structure having a chemical formula of ABO₃, and inorder to synthesize the ferroelectric material, the ferroelectricmaterial is prepared by milling, drying, grinding, and sieving a rawmaterial powder including A and B elements, and calcining the resultingproduct at a predetermined molar ratio in the same manner as in thetitanic acid compound. Here, a melting point of the ferroelectric needsto be lower than that of the titanic acid compound.

Step S110 is a step of mixing the prepared titanic acid compound withthe prepared ferroelectric. Since a ferroelectric 200 is entirely evenlydistributed in a titanic acid compound 100 during the mixing of thetitanic acid compound with the ferroelectric, the ferroelectric 200allows a titanic acid compound grain boundary to permeate into a liquidphase to form a grain boundary during the preparation of a dielectric,thereby preparing a dielectric having large sizes.

Step S120 is a step of sintering a mixture of the titanic acid compoundand the ferroelectric at a temperature equal to or more than a meltingpoint of the ferroelectric material. FIG. 2(c) illustrates a state wherethe mixture prepared in step S110 is sintered at a temperature equal toor more than a melting point of the ferroelectric material. When themixture is sintered at a temperature equal to or more than the meltingpoint of the ferroelectric, the ferroelectric is melted, and thus evenlydistributed among particles of the titanic acid compound 100.Preferably, as the sintering temperature, a temperature of approximately70% to approximately 90% of the melting point of the titanic acidcompound is suitable, but the sintering temperature is not limitedthereto.

Referring to FIG. 2, step S120 according to the present invention willbe specifically described.

The sintering step S120 is carried out by performing a process ofsintering a mixture of the titanic acid compound 100 and theferroelectric 200 under a reducing atmosphere and a subsequent heattreatment process under an oxidizing atmosphere.

FIG. 2(a) illustrates a form in which the titanic acid compound 100 andthe ferroelectric 200 are mixed through step S110. It can be confirmedthat the ferroelectric 200 is entirely evenly distributed in the titanicacid compound 100.

FIG. 2(b) illustrates a structure before the heat treatment temperaturereaches a melting point of the ferroelectric 200 in performing sinteringon the mixture of the titanic acid compound 100 and the ferroelectric200 under a reducing atmosphere. When a sintering is performed under areducing atmosphere, grain growth and densification of the titanic acidcompound (for example, ST and BT) occur. Compared with FIG. 2(a),particles of the titanic acid compound aggregate, and as a result, theform is changed, but is not yet a completely densified form, so that itcan be confirmed that empty spaces among the titanic acid compoundparticles are present. The ferroelectric 200 is present in the emptyspace. As the densification occurs, the empty spaces begin to be filled.

FIG. 2(c) illustrates a structure after the sintering temperature undera reducing atmosphere reaches the melting point of the ferroelectric200.

As the heat treatment temperature passes through the melting point ofthe ferroelectric, the phase of the ferroelectric is changed from thesolid phase to the liquid phase, and the ferroelectric flows into thegrain boundary. In this case, grain growth and densification of thetitanic acid compound (for example, ST and BT) actively occur, andsimultaneously, the ferroelectric 200 in a liquid state is present atthe grain boundary.

FIG. 2(d) illustrates a structure after the sintering under a reducingatmosphere is finished. When the temperature is lowered as the heattreatment is finished, that is, when the temperature is lowered to atemperature equal to or less than a melting point of the ferroelectric200, the ferroelectric 200 in a liquid state present at the grainboundary is changed into a solid state.

FIG. 2(e) illustrates a structure after the subsequent heat treatment isperformed under an oxidizing atmosphere. The reactions of DefectChemical Formulae 3 and 4 mentioned below at the grain boundary occur,and as a result, the insulation property of the grain boundary isreinforced.

Meanwhile, the heat treatment step may be performed by lowering thesample to a room temperature state after the sintering under a reducingatmosphere, creating an atmosphere as an oxidizing atmosphere of asubsequent heat treatment at room temperature, and then increasing thetemperature to the temperature of the subsequent heat treatment.However, after the sintering under a reducing atmosphere, the subsequentheat treatment may be performed by lowering the temperature to thesubsequent heat treatment temperature, which is not in a roomtemperature state, and changing the atmosphere into the oxidizingatmosphere. In the case of the former, the ferroelectric in a liquidphase is changed into a solid phase when the temperature is lowered toroom temperature after the completion of the sintering under a reducingatmosphere, and in the case of the latter, the ferroelectric in a liquidphase is changed into a solid phase when the temperature is lowered tothe subsequent heat treatment temperature after the sintering under areducing atmosphere.

The subsequent heat treatment may be performed at a temperature equal toor less than the melting point of the ferroelectric, and if thesubsequent heat treatment is performed at a temperature equal to or morethan the melting point, the ferroelectric at the grain boundary ispresent in a liquid phase even during the subsequent heat treatment, andis changed into a solid phase when the temperature is decreased to roomtemperature after the completion of the subsequent heat treatment.

As described above, for a titanic acid solid solution by the presentinvention, the process procedure is relatively simple because a two-stepheat treatment process under a reducing atmosphere and under anoxidizing atmosphere is performed unlike existing methods of performingsintering on a power molded body, applying oxide to the powder moldedbody, and performing a heat treatment.

A principle in which a grain boundary insulation-type dielectric isformed through the sintering under a reducing atmosphere and thesubsequent heat treatment under an oxidizing atmosphere will bespecifically described by exemplifying a strontium titanate compound. Adonor to be described in the present invention is a material having avalence larger than that of an element at a site to be added, and thedonor may be added by substituting a strontium (Sr) site with lanthanum(La) or substituting the strontium (Sr) site with an element having avalence of 3 or more, which corresponds to lanthanum (La).

When a donor having a large valence substitutes the original atomicposition, a positive charge is formed. In this case, aninfiltration-type oxygen ion, a positive ion vacancy, or an electron isadditionally formed and offsets a positive charge, and electricalneutrality is maintained.

Since it is difficult for an oxygen ion having a large ion size to enterthe lattices, the infiltration-type oxygen ion is minimally formed, andwhen a heat treatment is performed at normal pressure in which an oxygenpartial pressure is high, a positive ion vacancy is formed, and as theoxygen partial pressure is lowered, an electron rather than the positiveion vacancy is formed, thereby maintaining the electrical neutrality.

When the donor is added, the system is classified into three regionsaccording to the oxygen partial pressure. The case where the oxygenpartial pressure is very low, the case where the oxygen pressure is low,and the case where the oxygen partial pressure is high are shown inDefect Chemical Formulae 1 to 3, respectively in accordance with aKroger-Vink notation method. V indicates a vacancy, the subscript andthe superscript indicate an atom and an effective charge originallypositioned at a site to be substituted, respectively, ⋅ indicates apositive effective charge, and ′ indicates a negative effective charge.

Defect Chemical Formula 1 is a case where the oxygen partial pressure isvery lowas in a H₂ atmosphere, the electric conductivity is almostregardless of the concentration of a donor added, and as the oxygenpartial pressure is increased, the electric conductivity is decreased.

$\begin{matrix}{O_{0}->{{\frac{1}{2}O_{2}} + V_{O}^{-} + {2e^{\prime}}}} & \left( {{Defect}\mspace{14mu} {Chemical}\mspace{14mu} {Formula}\mspace{14mu} 1} \right)\end{matrix}$

When the oxygen partial pressure is low as in a 95N₂-5H₂ or nitrogenatmosphere, and the like, the electric conductivity is almost regardlessof the oxygen partial pressure, and since a donor additive forms freeelectrons according to Defect Chemical Formula 2, the electricconductivity is increased by the concentration of the additive.

$\begin{matrix}{{{{{Lo}_{2}{O_{3}\left( {{- 2}{SrO}} \right)}}->{{2\text{?}} + {2O_{O}} + {\frac{1}{2}O_{2}} + {2e^{\prime}}}}{{{Nb}_{2}{O_{5}\left( {{- 2}\text{?}O_{2}} \right)}}->{{2\text{?}} + {4O_{O}} + {\frac{1}{2}O_{2}} + {2e^{\prime}}}}{\text{?}\text{indicates text missing or illegible when filed}}}\mspace{11mu}} & \left( {{Defect}\mspace{14mu} {Chemical}\mspace{14mu} {Formula}\mspace{14mu} 2} \right)\end{matrix}$

When the oxidizing atmosphere, that is, the oxygen partial pressure(Po₂) is high, strontium ion vacancies are formed according to DefectChemical Formula 3, so that since free electrons are not formed and theelectric charges are compensated by ions to achieve the electricalneutrality, the electric conductivity is decreased.

$\begin{matrix}{{{{Lo}_{2}{O_{3}\left( {{- 3}{SrO}} \right)}}->{{2\text{?}} + V_{Sr}^{''} + {3O_{o}}}}{{2{Nb}_{2}{O_{5}\left( {{- 5}\text{?}O_{2}} \right)}}->{{4\text{?}} + \text{?} + {10O_{o}}}}} & \left( {{Defect}\mspace{14mu} {Chemical}\mspace{14mu} {Formula}\mspace{14mu} 3} \right) \\{{{{\frac{1}{2}O_{2}} + V_{O}^{-} + {2e^{\prime}}}->O_{O}}{\text{?}\text{indicates text missing or illegible when filed}}} & \left( {{Defect}\mspace{14mu} {Chemical}\mspace{14mu} {Formula}\mspace{14mu} 4} \right)\end{matrix}$

When the oxygen partial pressure is increased, oxygen in the atmospherediffuses into the sintered body, and as a result, a reaction occurs in aright direction where an oxygen vacancy and an electron meet together,and since oxygen diffuses more rapidly at the grain boundary than in thegrain, when a heat treatment is performed under an oxidizing atmosphere,an oxidation layer is formed only at the grain boundary and theresistance is increased. After the donor is added, a semiconductingsintered body is obtained by sintering under a reducing atmosphere(Defect Chemical Formulae 1 and 2), and then an insulating grainboundary is created by forming an oxidation layer only at the grainboundary (Defect Chemical Formulae 3 and 4) through a short subsequentheat treatment under an oxidizing atmosphere. Through sintering under areducing atmosphere and a subsequent heat treatment process under anoxidizing atmosphere, a grain boundary insulation-type capacitorconsisting of semiconducting particles and insulating grain boundariesmay be prepared. Hereinafter, a process of performing each step will bedescribed in detail through the examples.

FIG. 3 is a scanning electron microscope image illustrating particlesizes of a (Sr_(0.95)La_(0.05))Ti_(1.01)O₃ compound according to anexemplary embodiment of the present invention and a raw material powder.

A titanic acid compound (powder) and a ferroelectric powder are preparedby each calcining SrCO₃, La₂O₃, TiO₂ and KCO₃, NaCO₃, and Nb₂O₅ rawmaterial powders. As illustrated in FIG. 3,(Sr_(0.95)La_(0.05))Ti_(1.01)O₃ may be prepared by calcining La₂O₃ andTiO₂ powders. Hereinafter, (Sr_(0.95)La_(0.05))Ti_(1.01)O₃ is indicatedby ST, (Ba_(0.95)La_(0.05))Ti_(1.01)O₃ is indicated by BT,(K_(0.5)Na_(0.5))NbO₃ is indicated by KNN, and KNbO₃ is indicated by KN.

A method for preparing a high dielectric will be specifically describedby exemplifying a case where the titanic acid compound is a strontiumtitanate compound. A high dielectric consisting of a strontium titanatecompound is added in an amount of 1 to 5 mol % based on a site in whicha donor additive acts as a donor, and contains a titanium (Ti) site at amolar ratio of 1.00 to 1.01 based on the strontium (Sr) site. Aftersintering is performed under a reducing atmosphere for 1 to 2 hours byadding a ferroelectric having a melting point equal to or less than thesintering temperature of the present compound in an amount of 2 to 20mol % based on a strontium titanate (SrTiO₃) solid solution and mixingthe strontium titanate (SrTiO₃) solid solution, a subsequent heattreatment is performed under an oxidizing atmosphere for 30 minutes to 1hour.

In a donor, lanthanum (La) is added to a strontium (Sr) site or niobium(Nb) is added to a titanium (Ti) site, and the element is added in anamount of 1 to 5 mol % based on a position to be added. When theaddition concentration is less than 1 mol %, semiconducting particlesare not formed during sintering under a reducing atmosphere, and whenthe addition concentration is more than 5 mol %, the element added asthe donor may be precipitated at the grain boundary, so that an additionconcentration of 1 to 5 mol % is appropriate.

A ferroelectric having a melting point equal to or less than thesintering temperature of the present compound is melted during thesintering process of the compound, and thus enters between particles.When cooled after the sintering, the ferroelectric material remains atthe grain boundary as it is, and thus, acts as an insulator in a grainboundary insulation-type capacitor model, thereby increasing theapparent dielectric constant of the grain boundary.

The ferroelectric having a melting point equal to or less than thesintering temperature of the present compound is ABO₃, where A⁺=K, Na,K_(0.5)Na_(0.5) or a mixture thereof and B⁵⁺=Nb or Ta. Each meltingpoint of KNbO₃, KTaO₃, NaNbO₃, NbTaO₃ and K_(0.5)Na_(0.5)NbO₃ is 1,039°C., 1,370° C., 1412° C., 1780° C., and 1140° C. to 1420° C.,respectively [F. S. Galasso, “Perovskite and High Tc Superconductors,”p. 176 Gordon and Breach Science Publishers, New York (1986); B. Jaffe,“Piezoelectric Ceramics; Academic Press: London, UK (1971)].Accordingly, it is preferred that a ferroelectric added to the strontiumtitanate-based solid solution is KNbO₃, KTaO₃, NaTaO₃, orK_(0.5)Na_(0.5)NbO₃.

When the aforementioned ferroelectric is mixed with a strontium-basedsolid solution, in the case where the ferroelectric is present in anamount of less than 2 mol % based on the strontium-based solid solution,it is difficult for the ferroelectric to be uniformly present at all thegrain boundaries during the sintering, and in the case where theferroelectric is added in an amount of 20 mol % or more, the apparentdielectric constant (ε_(app)) is rather decreased as the thickness(d_(gb)) of the grain boundary is increased. Accordingly, theferroelectric to be added is appropriately present in an amount of 2 to20 mol % based on the strontium-based solid solution.

FIG. 4 is a flowchart illustrating a preparation method when a titanicacid compound is a barium titanate compound, in the grain boundaryinsulation-type dielectric according to an exemplary embodiment of thepresent invention.

A method for preparing a high dielectric (S200, S210, and S240) when thetitanic acid compound is a barium titanate compound (BaTiO₃) is similarto the aforementioned method for preparing a strontium titanatecompound.

However, as illustrated in FIG. 4, in the case of a high dielectricbased on a barium titanate compound, a step of adding TEOS to aferroelectric material and a titanic acid compound (S220) and a step ofsubjecting a mixture to which the TEOS is added to pre-heat treatment(prefiring) S230 may be additionally performed. Meanwhile, bariumtitanate is similar to the case of strontium titanate, and the donor maybe added by substituting a barium (Ba) site with lanthanum (La) orsubstituting the barium (Ba) site with an element having a valence of 3or more, which corresponds to lanthanum (La).

The difference between detailed process conditions of barium titanateand strontium titanate will be described through the Examples describedbelow.

The present invention will be specifically described through thefollowing Examples. However, the following Examples are provided forillustrative purposes only, and are not intended to limit the technicalscope of the present invention.

Example 1

In Example 1 according to the present invention, a dielectric having acomposition of (100-x)(Sr_(0.95)La_(0.05))Ti_(1.01)O₃-xKN (x=2, 5, 10,15, and 20) was prepared.

Raw material powders used to prepare these dielectrics are SrCO₃, La₂O₃,TiO₂, KCO₃, and Nb₂O₅. TiO₂ was allowed to have an average particle sizeof several hundred nanometers after all the processes were completed byusing a powder having a size of several tens of nanometers. La₂O₃ wasused as a donor additive, and KCO₃ and Nb₂O₅ were separately synthesizedand allowed to be positioned at the grain boundary during the sinteringprocess of strontium titanate to which the donor was added.

First, lanthanum (La) was positioned at an Sr site to act as a donor,and Sr, La, and Ti were put into a polyethylene bottle at a molar ratioof Sr:La:Ti=0.95:0.05:1.01 in order to slightly exceed Ti, and theresulting mixture was wet-milled with ZrO₂ balls for 24 hours by usingan alcohol solvent. After the product was dried on a hot plate until theslurry state was reached, the dried product was completely dried in anoven at 80° C., ground in an agate mortar, and then sieved.

When the melting point is lower than that of a strontium titanatecompound, K and Nb at a molar ratio of K:Nb=1:1 were milled, dried,ground, and sieved in the same manner as in the above-described methodin order to synthesize a ferroelectric KNbO₃ having a perovskitestructure.

A strontium titanate raw material mixed powder to which lanthanum (La)was added and a KNbO₃ raw material mixed powder were calcined at 1,300°C. for 4 hours and at 850° C. for 4 hours, respectively, by using anelectric furnace in the form of a box. Preferably, the raw materialmixture of strontium titanate may be calcined at 1,250 to 1,300° C., andthe KNbO₃ raw material mixed powder may be calcined at 800 to 900° C.

KNbO₃ was added at a molar ratio of 0, 5, 10, 15, and 20:1 to thesynthesized strontium titanate-based powder, and the resulting mixturewas put into a polyethylene bottle and wet-milled with zirconia (ZrO₂)balls for 24 hours by using an alcohol solvent. The resulting productwas milled, dried, ground, and sieved in the same manner as in theabove-described method. In order to manufacture a grain boundaryinsulation-type capacitor, a synthesized powder was injected into ametal mold having a diameter of 12 mm, primarily molded, andisostatically molded at 200 MPa for 5 minutes (preferably 5 to 10minutes).

Highly dense semiconducting particles were prepared by sintering theprepared molded body at 1,450° C. under a nitrogen atmosphere in avertical tube furnace for 2 hours. Thereafter, the grain boundary wasoxidized by performing a subsequent heat treatment under conditions of1,200° C., normal pressure, and 30 minutes in the vertical tube furnace,and a grain boundary insulation-type capacitor was prepared. Preferably,the heat treatment under the reducing atmosphere may be performed at1,300 to 1,500° C., and the heat treatment under the oxidizingatmosphere may be performed at 1,100 to 1,200° C.

Example 2

In Example 2 according to the present invention, a dielectric having acomposition of (100-x)ST-xKNN (x=2, 5, 10, 15, and 20) was prepared.

Raw material powders used to prepare these dielectrics are SrCO₃, La₂O₃,TiO₂, KCO₃, NaCO₃, and Nb₂O₅, and TiO₂ was allowed to have an averageparticle size of several hundred nanometers after all the processes werecompleted by using a powder having a size of several decade nanometers.

First, Sr, La, and Ti were put into a polyethylene bottle at a molarratio of Sr:La:Ti=0.95:0.05:1.01 in order to slightly exceed Ti, and theresulting mixture was wet-milled with zirconia (ZrO₂) balls for 24 hoursby using an alcohol solvent. After the product was dried on a hot plateuntil the slurry state was reached, the dried product was completelydried in an oven at 80° C., ground in an agate mortar, and then sieved.K and Nb at a molar ratio of K:Nb=1:1 were milled, dried, ground, andsieved in the same manner as in the above-described method in order tosynthesize K_(0.5)Na_(0.5)NbO₃.

A strontium titanate raw material mixed powder to which lanthanum (La)was added and a K_(0.5)Na_(0.5)NbO₃ raw material mixed powder werecalcined at 1,300° C. for 4 hours and at 900° C. for 4 hours,respectively, by using an electric furnace in the form of a box.Preferably, the raw material mixture of strontium titanate may becalcined at 1,250 to 1,300° C., and the K_(0.5)Na_(0.5)NbO₃ raw materialmixed powder may be calcined at 800 to 900° C.

K_(0.5)Na_(0.5)NbO₃ was added at a molar ratio of 0, 5, 10, 15, and 20:1to the synthesized strontium titanate-based powder, and the resultingmixture was put into a polyethylene bottle and wet-milled with zirconia(ZrO₂) balls for 24 hours by using an alcohol solvent. The resultingproduct was milled, dried, ground, and sieved in the same manner as inthe above-described method. In order to manufacture a grain boundaryinsulation-type capacitor, a synthesized powder was injected into ametal mold having a diameter of 12 mm, primarily molded, andisostatically molded at 200 MPa for 5 minutes.

Highly dense semiconducting particles were prepared by sintering theprepared molded body at 1,450° C. under a nitrogen atmosphere in avertical tube furnace for 2 hours. Thereafter, a grain boundaryinsulation-type capacitor was prepared by oxidizing the grain boundarythrough a subsequent heat treatment at 1,200° C. and normal pressure for30 minutes in the vertical tube furnace. Preferably, the heat treatmentunder the reducing atmosphere may be performed at 1,300 to 1,500° C.,and the heat treatment under the oxidizing atmosphere may be performedat 1,100 to 1,200° C.

FIG. 5 is a scanning electron microscope image illustrating the microstructures of the samples of (100-x)ST-xKNN according to an exemplaryembodiment of the present invention. The sintering under the reducingatmosphere and the subsequent heat treatment under the oxidizingatmosphere were performed under conditions of 1,450° C., N₂, and 2 hoursand under conditions of 1,200° C., normal pressure (air), and 30minutes, respectively.

Example 3

In Example 3 according to the present invention, a dielectric having acomposition of(100-x)(Ba_(0.95)La_(0.05))Ti_(1.01)O₃-xK_(0.5)Na_(0.5)NbO₃ (x=2, 5, 10,15, and 20) was prepared.

Raw material powders used to prepare these dielectrics are BaCO₃, La₂O₃,TiO₂, KCO₃, NaCO₃, and Nb₂O₅, and TiO₂ was allowed to have an averageparticle size of several hundred nanometers after all the processes werecompleted by using a powder having a size of several decade nanometers.

First, Ba, La, and Ti were put into a polyethylene bottle at a molarratio of Sr:La:Ti=0.95:0.05:1.01, and the resulting mixture waswet-milled with zirconia (ZrO₂) balls for 24 hours by using an alcoholsolvent. After the product was dried on a hot plate until the slurrystate was reached, the dried product was completely dried in an oven at80° C., ground in an agate mortar, and then sieved. K and Nb at a molarratio of K:Nb=1:1 were milled, dried, ground, and sieved in the samemanner as in the above-described method in order to synthesizeK_(0.5)Na_(0.5)NbO₃.

A barium titanate raw material mixed powder to which lanthanum (La) wasadded and a K_(0.5)Na_(0.5)NbO₃ raw material mixed powder were calcinedat 1,100° C. for 4 hours and at 900° C. for 4 hours, respectively byusing an electric furnace in the form of a box.

K_(0.5)Na_(0.5)NbO₃ was added at a molar ratio of 0, 5, 10, 15, and 20:1to the synthesized strontium titanate-based powder, 0 to 8 wt % oftetra-ethyl ortho silicate (TEOS) was added thereto, the resultingmixture was put into a polyethylene bottle and wet-milled with ZrO₂balls for 24 hours by using an alcohol solvent, and the resultingproduct was milled, dried, ground, and sieved in the same manner as inthe above-described method. When 0.5 to 8 wt % of TEOS is added thereto,dielectric characteristics may be improved. In order to manufacture agrain boundary insulation-type capacitor, a synthesized powder wasinjected into a metal mold having a diameter of 12 mm, primarily molded,and isostatically molded at 200 MPa for 5 minutes.

A molded body prepared prior to the sintering was subjected to apre-heat treatment (prefiring) process maintained under a 5H₂-95N₂atmosphere (reducing atmosphere) at 900° C. for 5 hours in a verticaltube furnace. By performing a pre-heat treatment (prefiring) prior tothe sintering, the density of the high dielectric may be increased, anda change in dielectric constant according to the frequency may be causedto occur less than before. The pre-heat treatment (prefiring) wasperformed under a 5H₂-95N₂ atmosphere, and thus may be performed underthe same atmosphere as the sintering under a reducing atmosphere. It ispossible to remove impurities introduced therein during the process ofpreparing the powder through the pre-heat treatment (prefiring).

After the pre-heat treatment (prefiring), highly dense semiconductingparticles were prepared by performing sintering under a 5H₂-95N₂atmosphere (reducing atmosphere) at 1,200 to 1,280° C. for 2 hours.Thereafter, a capacitor was prepared by oxidizing the grain boundarythrough a subsequent heat treatment at 1,000 to 1,100° C. under N₂ ornormal pressure (both under an oxidizing atmosphere) in a vertical tubefurnace for 30 minutes. In the heat treatment temperature andatmosphere, a desired reaction did not occur out of the temperaturerange, so that it was experimentally confirmed that in the prepareddielectric, dielectric characteristics and mechanical characteristicssignificantly deteriorated.

In the case of (Ba_(0.95)La_(0.05))Ti_(1.01)O₃, both N₂ and normalpressure are under an oxidizing atmosphere, but the difference indielectric characteristics was exhibited due to the difference in degreeof oxidation. When the heat treatment was performed under N₂, a highdielectric constant (high relative dielectric constant or highdielectric constant) was exhibited, whereas when the heat treatment wasperformed under normal pressure, a stable dielectric constant and a lowdielectric loss (loss tangent, tan δ) were exhibited in a wide frequencyregion of 100 Hz to 1 MHz.

FIG. 6 is a scanning electron microscope image of(Ba_(0.95)La_(0.05))Ti_(1.01)O₃ powder prepared through a calcinationprocess according to an exemplary embodiment of the present invention,and FIG. 7 is a scanning electron microscope image illustrating changesin micro structures according to the content of TEOS in90(Ba_(0.95)La_(0.05))Ti_(1.01)O₃-10KNN. The sintering under thereducing atmosphere and the subsequent heat treatment under theoxidizing atmosphere were performed under conditions of 1,200° C.,5H₂-95N₂, and 2 hours and under conditions of 1,100° C., normal pressure(air), and 30 minutes, respectively.

Analysis Example

After a cross-section of the strontium titanate compound prepared inExample 1 was cut, the compound was sequentially polished by using 6 μm,3 μm, and 1 μm diamond suspensions. Thereafter, thermal etching wasperformed at 1,000 to 1,100° C. in a vertical tube furnace for 1 minute,and then quenching was performed under air atmosphere, and in order toprevent a phenomenon in which electrons are accumulated on a surface ofa sample when electrons are injected thereinto, the surface was coatedwith osmium (Os) and observed by a scanning electronic microscopy.

After the upper and lower surfaces of the solid solution were polishedto 30 μm, a silver paste (Ag paste) was applied on one surface thereofby a silk screen technique, and organic materials included in the pastewere removed by drying the paste in an oven at 120° C. for 30 minutes.The same process was also repeated on the opposite surface. The relativedielectric constant and the dielectric loss were measured at 0.5 V and100 Hz to 5.5 MHz. The average particle size was obtained by SEM, an XRDanalysis was performed, and the relative dielectric constant and thedielectric loss were measured at 1 MHz.

FIG. 8A is a graph illustrating changes in relative dielectric constantvalues according to the frequency measured in(100-x)(Sr_(0.95)La_(0.05))Ti_(1.01)O₃-xKNN according to the presentinvention, and FIG. 8B is a graph illustrating changes in dielectricloss values.

Referring to FIGS. 8A and 8B and Table 1, it can be confirmed that eventhough the dielectric based on the strontium titanate compound has smallparticle sizes, a high relative dielectric constant and a low dielectricloss are exhibited in a high frequency region. The following Table 1shows a relative dielectric constant and a dielectric loss of thedielectric based on strontium titanate (SrTiO₃).

TABLE 1 composition and annealing condition dielectric characteristicsferroelectric 2nd firing dielectric Dielectric loss, donor materialcondition grain constant, ∈_(r) tan δ[%] sample La KNN temper- atmo-size 10 100 1 10 100 1 no. [mol %] [mol %] ature sphere [μm] kHz kHz MHzkHz kHz MHz 1 5 5 1200 air 0.6 5,390 4,900 4,760 28.7 6.3 2.4 2 5 101200 air 0.7 5,100 4,850 4,680 11.3 3.6 2.8 3 5 20 1200 air 0.7 6,1265,625 5,532 17.7 5.0 4.5

The dielectric based on the strontium titanate compound according to thepresent invention exhibited a high relative dielectric constant value of4,600 or more and also exhibited a dielectric loss value of less than 5%in a particle size of 0.3 μm to 1 μm in a high frequency region of 1MHz.

FIG. 9A is a graph illustrating changes in relative dielectric constantvalues according to the frequency measured in(100-x)(Sr_(0.95)La_(0.05))Ti_(1.01)O₃-xKN according to the presentinvention, and FIG. 9B is a graph illustrating changes in dielectricloss values.

Referring to FIG. 9, it can be confirmed that the relative dielectricconstant and dielectric loss values are constantly maintained without asignificant change, regardless of the frequency region. It was confirmedthat numerically, the relative dielectric constant was maintained withina change width of about 0 to 10, and the dielectric loss was maintainedwithin a change width of about 0% to 5%. Meanwhile, the particle size ofthe dielectric exhibited a value of 0.5 μm or less.

The following Table 2 shows the relative dielectric constant anddielectric loss of the (100-x)(Sr_(0.95)La_(0.05))Ti_(1.01)O₃-xKNdielectric, and the like.

TABLE 2 composition and annealing condition dielectric characteristicsferroelectric 2nd firing dielectric Dielectric loss, donor materialcondition grain constant, ∈_(r) tan δ[%] sample La KN temper- atmo- size100 1 10 100 1 100 1 10 100 1 no. [mol %] [mol %] ature sphere [μm] HzkHz kHz kHz MHz Hz kHz kHz kHz MHz 4 5 5 1200 air 0.3 144 141 140 137141 1.8 1.8 1.0 2.0 1.5 5 5 10 1200 air 0.3 199 195 193 191 193 1.1 2.14.4 2.3 0.2

FIG. 10A is a graph illustrating changes in relative dielectric constantvalues according to the frequency measured in (100-x)BT-xKNN in which anoxidizing atmosphere of a subsequent heat treatment process consists ofa N₂ oxidizing atmosphere in (100-x)BT-xKNN, and FIG. 10B is a graphillustrating changes in dielectric loss values.

The sintering under the reducing atmosphere was performed underconditions of 1,200° C., 5H₂-95N₂, and 2 hours, and the subsequent heattreatment under the oxidizing atmosphere was performed under conditionsof 1,050 or 1,100° C., N₂, and 30 minutes, in 90BT-10KNN and 80BT-20KNN.

Through the experiment, it was confirmed that in a particle size of 0.2μm to 1 μm in a high frequency region of 1 MHz, a relative dielectricconstant of 1,400 to 3,000 was exhibited, and a dielectric loss value of10 to 20% was exhibited.

FIG. 11A is a graph illustrating changes in relative dielectric constantvalues according to the addition of TEOS in 90BT-10KNN in which anoxidizing atmosphere of a subsequent heat treatment process consists ofa N₂ oxidizing atmosphere in (100-x)BT-xKNN, and FIG. 11B is a graphillustrating changes in dielectric loss values.

In 90BT-10KNN, the sintering under the reducing atmosphere and thesubsequent heat treatment under the oxidizing atmosphere were performedunder conditions of 1,200° C., 5H₂-95N₂, and 2 hours and underconditions of 1,100° C., N₂, and 30 minutes, respectively, and thecontent of tetraethyl orthosilicate (TEOS) was measured at 0, 0.5, and 2wt %. In comparison with FIG. 10A and FIG. 10B, in the case where apredetermined amount of TEOS was added, a relative dielectric constantof 1,900 to 3,200 and a dielectric loss of 10 to 18% were exhibited in afrequency region of 1 MHz.

FIG. 12A is a graph illustrating changes in relative dielectric constantvalues when (100-x)BT-xKNN is subjected to pre-heat treatment(prefiring) as a pre-step of sintering in 90BT-10KNN in which anoxidizing atmosphere of a subsequent heat treatment process consists ofa N₂ oxidizing atmosphere in (100-x)BT-xKNN, and FIG. 12B is a graphillustrating changes in dielectric loss values.

When a pre-heat treatment (prefiring) process prior to sintering wasperformed under the same conditions as the conditions described in FIG.11, the relative dielectric constant and the dielectric loss weremeasured. The pre-heat treatment (prefiring) was performed underconditions of 900° C., 5H₂-95N₂, and 5 hours.

As an example, when the blue color graph (a relative dielectric constantwhen 2 wt % of TEOS was added) in FIG. 11A is compared with the blackcolor graph (a relative dielectric constant when 2 wt % of TEOS wasadded) in FIG. 11A, it can be seen that when a pre-heat treatment(prefiring) prior to the sintering was performed, the change width ofthe dielectric constant is less than before, that is, the dielectricconstant is relatively constantly maintained, regardless of thefrequency region.

FIG. 13A is a graph illustrating changes in relative dielectric constantvalues of 90BT-10KNN according to the sintering temperature performedunder a reducing atmosphere when a subsequent heat treatment process isperformed under a N₂ oxidizing atmosphere in (100-x)BT-xKNN, and FIG.13B is a graph illustrating changes in dielectric loss values.

The sintering was performed at a temperature of 1,200° C., 1,250° C.,and 1,280° C. under a reducing atmosphere.

In the following Table 3, data in FIGS. 10 to 13 are summarized.

TABLE 3 composition and annealing condition ferroelectric additive 2ndfiring dielectric characteristics material material condition grain∈_(r) tan δ[%] sample KNN TEOS temper- atmo- size 1 1 1 1 no. [mol %][wt %] ature sphere [μm] kHz MHz kHz MHz 6 10 0 1100 N₂ 0.2 16,710 1,44063.9 15.5 7 10 0 1050 N₂ 0.2 19,620 1,440 63.6 15.3 8 20 0 1100 N₂ 0.317,980 1,870 50.8 14.1 9 20 0 1050 N₂ 0.3 17,420 1,900 71.9 11.8 10 100.5 1100 N₂ 0.2 10,980 1,970 61.5 11.9 11 10 2 1100 N₂ 0.2 12,280 2,80044.2 13.3 12 10 5 1100 N₂ 0.2 17,900 3,130 17.5 18.1 13 10 8 1100 N₂ 0.216,330 2,640 28.0 16.2

FIG. 14A is a graph illustrating changes in relative dielectric constantvalues according to the frequency measured in (100-x)BT-xKNN in which anoxidizing atmosphere of a subsequent heat treatment process consists ofa normal pressure (air) oxidizing atmosphere in (100-x)BT-xKNN, and FIG.14B is a graph illustrating changes in dielectric loss values.

The sintering under the reducing atmosphere was performed underconditions of 1,200° C., 5H₂-95N₂, and 2 hours, and the subsequent heattreatment under the oxidizing atmosphere was performed under conditionsof 1,050 or 1,100° C., normal pressure (air), and 30 minutes, in90BT-10KNN and 80BT-20KNN.

Referring to FIG. 14, it can be confirmed that the relative dielectricconstant and dielectric loss values are constantly maintained without asignificant change, regardless of the frequency region. It was confirmedthat numerically, in a particle size of 0.2 μm to 1 μm, the relativedielectric constant was maintained within a change width of about 0 toabout 20, and the dielectric loss was maintained within a change widthof about 0% to about 2%.

FIG. 15A is a graph illustrating changes in relative dielectric constantvalues according to the addition of TEOS in 90BT-10KNN in which anoxidizing atmosphere of a subsequent heat treatment process consists ofa normal pressure (air) oxidizing atmosphere in (100-x)BT-xKNN, and FIG.15B is a graph illustrating changes in dielectric loss values.

In 90BT-10KNN, the sintering under the reducing atmosphere and the heattreatment under the oxidizing atmosphere were performed under conditionsof 1,200° C., 5H₂-95N₂, and 2 hours and under conditions of 1,100° C.,normal pressure (air), and 30 minutes, respectively, and the content oftetraethyl orthosilicate (TEOS) was measured at 0, 0.5, and 2 wt %. Incomparison with FIG. 14A and FIG. 14B, in the case where a predeterminedamount of TEOS is added, it can be confirmed that in a frequency regionof 1 MHz, a less change width is maintained than in a frequency regionthan in a range where the relative dielectric constant and thedielectric loss are high.

FIG. 16A is a graph illustrating changes in relative dielectric constantvalues when (100-x)BT-xKNN is subjected to pre-heat treatment(prefiring) as a pre-step of sintering in 90BT-10KNN in which anoxidizing atmosphere of a subsequent heat treatment process consists ofa normal pressure (air) oxidizing atmosphere in (100-x)BT-xKNN, and FIG.16B is a graph illustrating changes in dielectric loss values.

When a pre-heat treatment (prefiring) process prior to sintering wasperformed under the same conditions as the conditions described in FIG.15, the relative dielectric constant and the dielectric loss weremeasured. The pre-heat treatment (prefiring) was performed underconditions of 900° C., 5H₂-95N₂, and 5 hours.

When compared with FIG. 15, it can be confirmed that when the pre-heattreatment (prefiring) prior to sintering is performed, a less changewidth is maintained than in a range where the relative dielectricconstant and the dielectric loss are high.

FIG. 17A is a graph illustrating changes in relative dielectric constantvalues of 90BT-10KNN according to the sintering temperature under areducing atmosphere when a subsequent heat treatment process isperformed under a normal pressure (air) oxidizing atmosphere in(100-x)BT-xKNN, and FIG. 17B is a graph illustrating changes indielectric loss values.

The sintering was performed at a temperature of 1,200° C., 1,250° C.,and 1,280° C. under a reducing atmosphere.

In the following Table 4, data in FIGS. 14 to 17 are summarized.

TABLE 4 composition and annealing condition dielectric characteristicsferroelectric additive 2nd firing dielectric Dielectric loss, materialmaterial condition grain constant, ∈_(r) tan δ[%] sample KNN TEOStemper- atmo- size 100 1 10 100 1 100 1 10 100 1 no. [mol %] [wt %]ature sphere [μm] Hz kHz kHz kHz MHz Hz kHz kHz kHz MHz 14 10 0 1100 air0.2 297 297 296 294 300 1.6 1.5 0.9 1.8 0.4 15 10 0 1050 air 0.2 329 315314 311 317 1.8 1.6 1.1 1.8 0.8 16 20 0 1100 air 0.3 387 375 378 378 3811.1 1.6 1.2 0.3 0.7 17 20 0 1050 air 0.3 370 363 365 366 368 1.2 1.4 1.20.4 1.1 18 10 0.5 1100 air 0.2 438 434 433 423 433 0.5 0.5 0.6 1.9 0.719 10 2 1100 air 0.2 511 509 506 503 504 0.5 0.6 0.7 0.5 1.0 20 10 51100 air 0.2 595 586 583 580 582 1.4 0.9 0.7 0.4 0.4 21 10 8 1100 air0.2 444 441 439 437 439 1.1 0.5 0.6 0.3 0.3

Representative exemplary embodiments of the present invention have beendescribed in detail, but it is to be understood by a person withordinary skill in the art to which the present invention pertains thatvarious modifications are possible with respect to the above-describedexample within the limitation without departing from the scope of thepresent invention. Therefore, the right scope of the present inventionshould not be defined by being limited to the described Examples, andshould be defined by not only the claims to be described below, but alsothose equivalent to the claims.

What is claimed is:
 1. A method for preparing a grain boundaryinsulation-type dielectric, the method comprising: obtaining a titanicacid compound and a ferroelectric having a value less than a meltingpoint of the titanic acid compound; obtaining a mixture by adding theferroelectric material to the titanic acid compound; and sintering themixture at a temperature equal to or more than a melting point of theferroelectric material.
 2. The method of claim 1, wherein the titanicacid compound is SrTiO₃ or (Sr_(x)A_(y))Ti_(z)O₃ where A is an elementhaving a valence of 3 or more, 0.95≤x≤0.99, 0.01≤y≤0.05, 1.00≤z≤1.01,and x+y=1.
 3. The method of claim 2, wherein the sintering comprisessintering under a reducing atmosphere and a subsequent heat treatmentunder an oxidizing atmosphere, and the subsequent heat treatment underthe oxidizing atmosphere is performed under normal pressure.
 4. Themethod of claim 1, wherein the titanic acid compound is BaTiO₃ or(Ba_(x)A_(y))Ti_(z)O₃ where A is an element having a valence of 3 ormore, 0.95≤x≤0.99, 0.01≤y≤0.05, 1.00≤z≤1.01, and x+y=1.
 5. The method ofclaim 4, further comprising adding tetraethyl orthosilicate (TEOS) tothe mixture of the titanic acid compound and the ferroelectric.
 6. Themethod of claim 4, wherein further comprising subjecting the mixture topre-heat treatment (prefiring) prior to the sintering.
 7. The method ofclaim 4, wherein the sintering comprises sintering under a reducingatmosphere and a subsequent heat treatment under an oxidizingatmosphere, and the subsequent heat treatment under the oxidizingatmosphere is performed under a N₂ atmosphere or normal pressure.
 8. Themethod of claim 1, wherein an addition ratio of the ferroelectric is 2to 20 mol % based on the titanic acid compound.
 9. The method of claim1, wherein the ferroelectric material is ABO₃, which has a perovskitestructure where A is any one of K, Na and K_(0.5)Na_(0.5), and B is anyone of Nb and Ta.
 10. A grain boundary insulation-type dielectricwherein a ferroelectric having an ABO₃ structure in(Sr_(x)La_(y))Ti_(z)O₃ which is a strontium titanate compound isdistributed at a grain boundary of the strontium titanate compound where0.95≤x≤0.99, 0.01≤y≤0.05, 1.00≤z≤1.01, x+y=1, A is any one of K, Na andK_(0.5)Na_(0.5), and B is any one of Nb and Ta.
 11. The grain boundaryinsulation-type dielectric of claim 10, wherein the grain boundaryinsulation-type dielectric has an average particle size of 0.3 μm to 1μm.
 12. The grain boundary insulation-type dielectric of claim 10,wherein when the ferroelectric is K_(0.5)Na_(0.5)NbO₃, the grainboundary insulation-type dielectric has a relative dielectric constantof 4,500 to 6,000 and a dielectric loss of 2 to 5% in a frequency regionof 1 MHz or more.
 13. The grain boundary insulation-type dielectric ofclaim 10, wherein when the ferroelectric is KNbO₃, the grain boundaryinsulation-type dielectric has a change width in relative dielectricconstant maintained at 0 to 10 and a change width in dielectric lossmaintained at 0 to 5%, regardless of the frequency region.
 14. A grainboundary insulation-type dielectric wherein a ferroelectric having anABO₃ structure in (Ba_(x)La_(y))Ti_(z)O₃ which is a barium titanatecompound is distributed at a grain boundary of the barium titanatecompound where 0.95≤x≤0.99, 0.01≤y≤0.05, 1.00≤z≤1.01, x+y=1, A is anyone of K, Na and K_(0.5)Na_(0.5), and B is any one of Nb and Ta.
 15. Thegrain boundary insulation-type dielectric of claim 14, wherein the grainboundary insulation-type dielectric has an average particle size of 0.2μm to 1 μm.
 16. The grain boundary insulation-type dielectric of claim14, wherein the grain boundary insulation-type dielectric has a relativedielectric constant of 1,400 to 3,200 and a dielectric loss of 10 to 20%in a frequency region of 1 MHz or more.
 17. The grain boundaryinsulation-type dielectric of claim 14, wherein in the grain boundaryinsulation-type dielectric, a change width in relative dielectricconstant is maintained at 0 to 20 and a change width in dielectric lossis maintained at 0 to 2%, regardless of the frequency region.
 18. Themethod of claim 10, wherein a ratio of the ferroelectric in the grainboundary insulation-type dielectric is 2 to 20 mol % based on thetitanic acid compound.
 19. The method of claim 14, wherein a ratio ofthe ferroelectric in the grain boundary insulation-type dielectric is 2to 20 mol % based on the titanic acid compound.